How can you assume resources scarcity if energy isn't conserved? At this point all bets are off. How do we know that traveling closer in the same time takes le

How can you assume resources scarcity if energy isn't conserved? At this point all bets are off. How do we know that traveling closer in the same time takes less energy than traveling further? We can't assume that anymore. And so on. This is my problem with most sci-fi. They take some crazy assumptions and ignore most of the consequences so the world is recognizable. This is like people in ancient Greece hearing that one city in 21st century has more population than the whole world in their time and wondering how we feed our horses :)